Reintroductions of rare plants require detailed knowledge of habitat requirements, species interactions, and restoration techniques. Thus, incremental experimentation over many years may be required to develop adequate knowledge and techniques for successful reintroduction. To determine drivers of extinction in historical reintroductions of a federally endangered perennial ( Astragalus bibullatus ), we developed a reintroduction experiment to disentangle the relative importance of habitat quality, herbivores, and restoration technique on reintroduction success. In a factorial design, we manipulated access to vertebrate herbivores across different habitat types (mesic ecotone vs. xeric barren), and used founder populations comprised of more transplants and genetic sources than previous reintroduction attempts. In mesic ecotones where historical reintroductions failed, excluding herbivores, thinning woody encroachment to improve habitat quality, outplanting across a greater array of microhabitats, and increasing founder population size did not improve demographic rates over previous attempts. Compared to mesic ecotones, transplant survival rates and cumulative fruit production were more than two and ten times greater, respectively, in a xeric barren ecotone characterized by open, grassy, and dry microenvironmental conditions. Across all sites, herbivores decreased probabilities of survival and flowering of larger adult plants. Flowering rates were 80% greater inside relative to outside herbivore exclusion cages. Over a four-year period, only a single uncaged plant produced fruit. Our study demonstrates that habitat quality and vertebrate herbivory are key drivers of long-term persistence in rare plant reintroductions. Using incremental experiments that build on previous knowledge gained from long-term monitoring can improve reintroduction outcomes.

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Inferring habitat requirements of rare plants can be challenging when the few remaining populations occur in sites with divergent successional states. In island-like rock outcrop systems within forest landscapes, edaphic conditions are assumed to modulate successional patterns, but changes to disturbance regimes in the landscape matrix could alter ecotone microenvironments over time. We used demographic surveys and controlled experiments with a dispersal-limited and endangered ecotonal plant, Astragalus bibullatus, to test the hypothesis that woody encroachment from the forest matrix threatens rare plants in globally imperiled limestone cedar glades. Tree canopy cover was more important than edaphic conditions or ground-layer structure for explaining variation in demographic structure. As tree canopy cover increased, stem densities, flowering, and seed production declined. Over three-years, per plant inflorescence production, fruit production, and fruit set were markedly greater in open microhabitats than in edge or closed microhabitats. In contrast, seedling densities peaked in edge and closed microhabitats. Seedling recruitment and seed production were spatially decoupled across a canopy cover gradient, suggesting that shaded sites historically had lower tree canopy cover. Shade reduced growth rates and biomass of seedlings and adults under nonlimiting moisture conditions. Although A. bibullatus can persist in degraded ecotones using multiple demographic strategies, growth, flowering and seed production depend on open microhabitats. Our results demonstrate that woody encroachment effects in temperate grasslands extend to island-like rock outcrop systems with unique edaphic conditions. Conservation and recovery programs with rare ecotonal plants should integrate restoration of historical disturbance regimes in the landscape matrix.